1. Field of the Invention
The present invention relates to the production of stencils for screen printing.
2. Related Background Art
The production of screen printing stencils is generally well known to those skilled in the art.
One method, referred to as the xe2x80x9cdirect methodxe2x80x9d of producing screen printing stencils involves the coating of a liquid light-sensitive emulsion directly onto the screen mesh. After drying, the entire screen is exposed to actinic light through a film positive held in contact with the screen in a vacuum frame. The black portions of the positive do not allow light to penetrate to the emulsion which remains soft in those areas. In the areas which are exposed to light, the emulsion hardens and becomes insoluble, so that, after washing out with a suitable solvent, the unexposed areas allow ink to pass through onto a substrate during a subsequent printing process.
Another method, referred to as the xe2x80x9cdirect/indirect methodxe2x80x9d, involves contacting a film, consisting of a pre-coated unsensitised emulsion on a support base, with the screen mesh by placing the screen on top of the flat film. A sensitised emulsion is then forced across the mesh from the opposite side, thus laminating the film to the screen and at the same time sensitising its emulsion. After drying, the base support is peeled off and the screen is then processed in the same way as in the direct method.
In the xe2x80x9cindirect methodxe2x80x9d a film base is pre-coated with a pre-sensitised emulsion. The film is exposed to actinic light through a positive held in contact with the coated film. After chemical hardening of the exposed emulsion, the unexposed emulsion is washed away. The stencil produced is then mounted on the screen mesh and used for printing as described above for the direct method.
In the xe2x80x9ccapillary direct methodxe2x80x9d a pre-coated and pre-sensitised film base is adhered to one surface of the mesh by the capillary action of water applied to the opposite surface of the mesh. After drying, the film is peeled off and the screen then processed and used as described for the direct method.
In addition to the above methods, hand-cut stencils can be used. These are produced by cutting the required stencil design into an emulsion coating on a film base support. The cut areas are removed from the base before the film is applied to the mesh. The emulsion is then softened to cause it to adhere to the mesh. After drying, the base is peeled off. The screen is then ready for printing. This method is suitable only for simple work.
One problem generally associated with the prior art methods is that many steps are necessary to produce the screen, thus making screen production time-consuming and labour-intensive.
Another problem is that normal lighting cannot be used throughout the screen production process in any of the methods except hand cutting. This is because the stencil materials are light-sensitive. In addition, it is necessary to provide a source of actinic (usually UV) light for exposing the stencil. This usually incurs a penalty of initial cost, space utilisation and ongoing maintenance costs.
Other methods of preparing printing screens are available. CA-A-2088400 (Gerber Scientific Products. Inc.) describes a method and apparatus in which a blocking composition is ejected directly onto the screen mesh surface in a pre-programmed manner in accordance with data representative of the desired image. The blocking composition directly occludes sections of the screen mesh to define the desired stencil pattern.
EP-A-0492351 (Gerber Scientific Products, Inc.) describes a method wherein an unexposed light-sensitive emulsion layer is applied to a screen mesh surface and a graphic is directly ink-jet printed on the emulsion layer by means of a printing mechanism to provide a mask through which the emulsion is exposed before the screen is further processed.
Both the above methods require the use of very specialised equipment which incurs a certain cost as well as imposing restrictions arising from the limitations of the equipment, in particular in terms of the size of screen and its resolution. The second method also requires sensitised films and emulsions, requiring exposure units and vacuum frames.
Ink-jet printers operate by ejecting ink onto a receiving substrate in controlled patterns of closely spaced ink droplets. By selectively regulating the pattern of ink droplets, ink jet printers can be used to produce a wide variety of printed materials, including text, graphics and images on a wide range of substrates. In many ink jet printing systems, ink is printed directly onto the surface of the final receiving substrate. An ink jet printing system where an image is printed on an intermediate image transfer surface and subsequently transferred to the final receiving substrate is disclosed in U.S. Pat. No. 4,538,156 (ATandT Teletype Corp.). Furthermore, U.S. Pat. No. 5,380,769 (Tektronix Inc.) describes reactive ink compositions containing at least two reactive components, a base ink component and a curing component, that are applied to the receiving substrate separately. The base ink component is preferably applied to the receiving surface using ink jet printing techniques and, upon exposure of the base ink component to the curing component, a durable, crosslinked ink is produced.
EP-A-0108509 (Pilot Man-Nen-Hitsu KK) describes a process in which a coating on a porous support is selectively chemically solubilised and then washed away.
EP-A-0770552 (Riso Kagaku Corporation) describes a machine in which a porous substrate forms the cylindrical surface of a drum. The substrate has a coating layer which is selectively solubilised prior to ink being passed outwardly through the substrate to be transferred to a substrate.
GB-A-180778 (Carter) describes a stencil paper which is coated with varnish prior to application of an ink. The ink and the varnish below is then washed away to form a stencil for a rotary duplicating machine.
Our co-pending Application PCT/GB97/01881 (WO99/02344, the content of which is incorporated herein by reference, describes a method of producing a screen-printing stencil which requires the image applied to a receptor element to be a negative image (this known as xe2x80x9cnegative workingxe2x80x9d). In the present application, a method is disclosed which uses a positive image (xe2x80x9cpositive workingxe2x80x9d). Each method has its advantages compared with the other, depending upon the circumstances of use.
One object of the present invention is to make screen-printing stencil production less time-consuming and labour-intensive.
Another object is to allow normal lighting to be used throughout the stencil production process and to avoid both the problems of prior art stencil materials which are light-sensitive and also the need to provide a source of actinic (usually UV) light for exposing the stencil.
The present invention provides a method of producing a screen-printing stencil having open areas and blocked areas for, respectively, passage and blocking of a printing medium, the method comprising:
providing a receptor element comprising an optional support base and an image-receiving layer comprising a first substance,
applying imagewise to the image-receiving layer a second substance in areas corresponding to the open areas of the stencil to be produced,
bringing the image-receiving layer into contact with a third substance applied in a layer to a screen-printing screen,
causing or allowing chemical reaction to take place to form on the screen a stencil-forming layer having areas of relatively higher and relatively lower solubility corresponding to the open and the blocked stencil areas respectively, and
washing away the stencil-forming layer in the higher solubility areas, thereby to produce the screen-printing stencil,
the first, second and third substances being such that the said chemical reaction takes place as stated.
Within this overall scope, four particular methods can be identified as preferred aspects of the invention, although the invention is not limited to these four preferred aspects.
In a first preferred aspect, the invention provides a method of producing a screen-printing stencil having open areas and blocked areas for, respectively, passage and blocking of a printing medium, the method comprising:
providing a receptor element comprising an optional support base and an image-receiving layer comprising a chemical agent reactive with a stencil-forming chemical agent,
applying imagewise to the image-receiving layer an inhibitor for the reaction between the chemical agent and the stencil-forming agent,
the areas to which the inhibitor is applied corresponding to the open areas of the stencil to be produced,
applying a composition comprising the stencil-forming chemical agent to a screen-printing screen,
bringing the image-receiving layer of the receptor element into contact with the stencil-forming composition to allow the uninhibited chemical agent to react to produce on the screen a stencil-forming layer having areas of lower solubility corresponding to the blocked stencil areas and areas of higher solubility corresponding to the open stencil areas,
removing any unreacted part of the receptor element, and
washing away the stencil-forming chemical agent in the higher solubility areas, thereby to produce the screen-printing stencil.
In a second preferred aspect, the invention provides a method of producing a screen-printing stencil having open areas and blocked areas for respectively passage and blocking of a printing medium, the method comprising:
providing a receptor element comprising an optional support base and an image-receiving layer,
applying imagewise to the image-receiving layer a reaction inhibitor,
the areas to which the inhibitor is applied corresponding to the open areas of the stencil to be produced,
applying to a screen-printing screen a composition comprising a stencil-forming chemical agent, a chemical agent reactive therewith and a temporary inhibitor for the reaction therebetween, the said reaction being inhibited by the reaction inhibitor,
bringing the image-receiving layer of the receptor element into contact with the composition applied to the screen-printing screen,
causing or allowing the temporary inhibitor to leave the composition applied to the screen and thereby allow the stencil-forming chemical agent and the chemical agent reactive therewith to react where not inhibited by the reaction inhibitor and thereby produce on the screen a stencil-forming layer having areas of lower solubility corresponding to the said blocked areas and areas of higher solubility corresponding to the open stencil areas, removing any unreacted part of the receptor element, and
washing away unreacted composition from the higher solubility areas, thereby to produce the screen-printing stencil.
In a third preferred aspect, the invention provides a method of producing a screen-printing stencil having open areas and blocked areas for respectively passage and blocking of a printing medium, the method comprising:
providing a receptor element comprising an optional support base and an image-receiving layer comprising a chemical agent reactive with a stencil-forming chemical agent,
applying imagewise to the image-receiving layer a masking agent which prevents migration of the chemical agent from the image-receiving layer,
the areas to which the masking agent is applied corresponding to the open areas of the stencil to be produced,
applying a composition comprising the stencil-forming chemical agent to a screen-printing screen,
bringing the image-receiving layer of the receptor element into contact with the stencil-forming chemical agent to allow the reactive chemical agent in areas not masked by the masking agent and the stencil-forming chemical agent to react to produce on the screen a stencil-forming layer having areas of lower solubility corresponding to the blocked areas and areas of higher solubility corresponding to the open stencil areas,
removing the unreacted part of the receptor element, and
washing away the second stencil-forming chemical agent in the higher solubility areas, thereby to produce the screen-printing screen.
In a fourth preferred aspect, the invention provides a method of producing a screen-printing stencil having open areas and blocked areas for, respectively, passage and blocking of a printing medium, the method comprising:
providing a receptor element comprising an optional support base, and an image-receiving layer comprising at least one component of a polymerisation system,
applying imagewise to the image-receiving layer an inhibitor for the polymerisation,
the areas to which the inhibitor is applied corresponding to the open areas of the stencil to be produced,
applying a stencil-forming composition comprising further components, including polymerisable material, of the polymerisation system to a screen-printing screen,
bringing the image-receiving layer of the receptor element into contact with the stencil-forming composition to allow the polymerisation to take place where not inhibited by the inhibitor to produce on the screen a stencil-forming layer having areas of lower solubility corresponding to the blocked stencil areas and areas of higher solubility corresponding to the open stencil areas,
removing any unreacted part of the receptor element, and
washing away the stencil-forming composition in the higher solubility areas, thereby to produce the screen-printing stencil.
In any of these aspects, the image-receiving layer of the receptor element may comprise a substance which takes part in the reaction between the stencil-forming chemical agent and the chemical agent reactive therewith, whereby the chemical agent of the image-receiving layer forms a part of the stencil-forming layer of the stencil produced after washing away unreacted composition from the higher solubility areas.
The invention further provides a coated film product for use in the production of a screen-printing stencil, the product comprising an optional support base and an image-receiving layer which comprises one or more of the following active agents:
boric acid;
a boron salt, for example Group I and Group II metal borates;
an aldehyde, for example formaldehyde:
a dialdehyde, for example glyoxal and glutaraldehyde, optionally with a mineral acid; and
transition metal compounds, for example iron (III), zirconium and titanium salts and chromium compounds, for example, pentahydroxy (tetradecanoate) dichromium and its derivatives.
Such a film product is particularly useful in methods according to the first and third preferred aspects of the invention but is not limited to such use.
The invention further provides a coated film product for use in the production of a screen-printing stencil, the product comprising an optional support base and an image-receiving layer which comprises at least one component of a free-radical generating system.
Preferably, the image-receiving layer further comprises a compound capable of taking part in a free-radical polymerisation process.
Such a film product is particularly useful in methods according to the fourth preferred aspect of the invention but is not limited to such use.
The invention also provides a pre-filled cartridge for a dropwise application device such as an ink-jet printer of plotter, the cartridge containing one or more of the following, optionally in a suitable liquid solvent or carrier:
a substance capable of reacting with boric acid or a boron salt, for example a Group I or Group II metal borate, in order to produce an insoluble borate;
a chelating agent, preferably an alkylene diaminetetraacetic acid, for example ethylenediaminetetraacetic acid, or a derivative thereof, or a mixture of two or more such chelating agents; and
an aromatic polyol, preferably an hydroxy-substituted benzene derivative, for example pyrogallol or catechol.
Such a cartridge is particularly useful in any of the four stated preferred aspects of the invention but is not limited to such use.
Still further, the invention provides a composition for use in coating a screen-printing mesh in the preparation of a screen-printing stencil, the composition comprising at least one compound capable of taking part in a free-radical polymerisation process to produce a hardened stencil material, and at least one component of a free-radical generating system.
Preferably, the composition includes a further substance which is incorporated into the polymerisation product upon polymerisation. The further substance may be, for example, polyvinyl alcohol.
Such a composition is particularly useful in methods according to the fourth preferred aspect of the invention but is not limited to such use.
Yet further, the invention provides a composition for use in coating a screen-printing mesh in the preparation of a screen-printing stencil, the composition comprising at least one compound capable of taking part in an ion-bridged cross-linking reaction to produce a hardened stencil layer on the mesh, a source of cross-linking ions, and a temporary inhibitor for the polymerisation reaction.
Such a composition is particularly useful in methods according to the second aspect of the invention but is not limited to such use.
According to the present invention, the stencil is formed by chemical means, without the need to use either special lighting conditions or actinic radiation.
Also, it is possible to carry out the invention with reduced expenditure in time and labour, compared with known processes.
The method of the invention is positive working: the material which is applied imagewise is applied in areas which correspond to the open areas of the eventual stencil.
When dropwise application is employed, the application is preferably controlled according to data encoding the desired pattern of blocked and open areas of the stencil to be produced. This control is conveniently by a computer, for example a personal computer. Thus, data representative of the desired output pattern can be input to a controller as pre-recorded digital signals which are used by the ejection head to deposit or not deposit the material applied imagewise as the head scans the surface of the receptor element. The invention is not however restricted to dropwise application of the material applied imagewise: other methods of application will achieve the same essential end, for example, the material applied imagewise could be applied with a hand-held marker pen.